The switched mode power supply (SMPS) is a well-known type of power converter having a diverse range of applications by virtue of its small size and weight and high efficiency, for example in personal computers and portable electronic devices such as cell phones. A SMPS achieves these advantages by switching one or more switching elements such as power MOSFETs at a high frequency (usually tens to hundreds of kHz), with the frequency or duty cycle of the switching being adjusted using a feedback signal to convert an input voltage to a desired output voltage. A SMPS may take the form of a rectifier (AC/DC converter), a DC/DC converter, a frequency changer (AC/AC) or an inverter (DC/AC).
If the power output capability of a single SMPS is not sufficient for a given application, it may be possible to connect multiple SMPSs in parallel to supply power to a load via a common output rail. SMPSs that are not output voltage-regulated (especially diode-rectified DC/DC converters) are well-suited to load current sharing arrangements of this kind, since the parasitic resistance in the power train of each SMPS usually causes the converter's output voltage to decrease as the converter's output current increases, i.e. a natural output voltage droop is present. The droop exhibited by each SMPS facilitates current sharing with other SMPSs in the system by effectively regulating the output voltage of the SMPS so as to counter any imbalance between the SMPS's output current and the output currents of the other SMPSs in the system. Thus, the voltage droop characteristic allows the paralleled SMPSs to share the task of supplying current to their load evenly. Load sharing in such a system of paralleled SMPSs may be improved by increasing the amount of voltage droop exhibited by each SMPS, and taking care to set the initial setting of each SMPS (i.e. output voltage at zero load current) appropriately.
However, there are many SMPS applications in which the SMPS output voltage to be supplied to load circuitry (e.g. a CPU) must be regulated so as to remain within a (usually very narrow) voltage band. Output voltage regulation may be achieved, for example, by feeding a signal indicative of the SMPS output voltage back to a pulse width modulation (PWM) controller of the SMPS, which monitors the feedback signal and adjusts the switching duty cycle of the SMPS's switching element(s) so as to maintain the output voltage at a predetermined value, regardless of the load current level. Alternatively, a switching frequency controller may be used instead of a PWM controller, to control the switching frequency of the SMPS's switching element(s) so as to maintain the output voltage at a predetermined value, regardless of the load current level. Owing to their wide availability, it is also often desirable to run voltage-regulated SMPSs in parallel to feed load circuits having high power demands. However, the absence of a natural output voltage droop makes it difficult to maintain even current sharing among the SMPSs in such parallel systems, and it therefore becomes necessary to introduce some form of artificial voltage droop in the SMPSs. A simplified example of how a regulated SMPS may be provided with artificial droop will now be explained with reference to FIG. 1.
FIG. 1 is a schematic of a conventional SMPS 100, which comprises a switch network 110 having one or more active switching devices (e.g. MOSFETs) that are controlled to switch between conducting (“ON”) and non-conducting (“OFF”) states with a switching duty cycle set by a PWM controller 120. The PWM controller 120 determines the duty cycle based on a feedback signal indicative of the SMPS output voltage, Vout, so that the output voltage Vout is maintained at a substantially constant value for any given SMPS current load. The PWM controller 120 is controlled to provide a voltage droop by a voltage droop control circuit 130. More specifically, the voltage droop control circuit 130 generates, based on a signal indicative of the SMPS output current that is generated by a current sensing circuit 140, a droop control signal 150 for adjusting an output voltage set-point (i.e. an output voltage target value) that is used by the PWM controller 120 to regulate Vout.
The current sensing circuit 140 may simply be configured to measure the voltage drop across a dedicated shunt resistor connected to the output of the SMPS 100, such that the voltage droop control circuit 130 receives said voltage drop as the signal indicative of the SMPS output current, Iout. However, the use of a current shunt to measure Iout has the draw-backs of degrading the thermal coupling of the power train 110 to the output pin(s) of the SMPS 100, and decreasing SMPS efficiency through resistive losses.
In switched mode power supplies where an output choke is present (e.g. in Forward-topology DC/DC converters, among others), these problems may be avoided by making a “lossless” current measurement as described, for example, in “A Simple Current-Sense Technique Eliminating a Sense Resistor” (Linfinity Application Note AN-7, Rev. 1.1 07/1998). In these cases, the current sensing circuit 140 and the voltage droop control circuit 130 may be configured as shown schematically in FIG. 2, while the PWM controller 120 may be provided in the form shown schematically in FIG. 3.
In the example of FIG. 2, the current sensing circuit 140 comprises a series combination of a resistor R0 and capacitor C, which is connected to the upper output rail of the SMPS 100, in parallel with the output choke having an inductance L and parasitic resistance RL. The voltage droop control circuit 130 comprises an operational amplifier (“op-amp”) 131 whose inverting (−) and non-inverting (+) inputs are connected across capacitor C via resistors R1 and R2, respectively. Being an active device, the op-amp 131 needs to receive power from a voltage source (not shown) in order to be able to generate a signal VA, which is based on the signals received at the inverting (−) and non-inverting (+) inputs. The signal VA is output to the PWM controller 120 shown in FIG. 3, and is also fed back to the inverting input via resistor R3. In FIG. 2, the signal indicative of the output current is superimposed with a large square wave from the switch network 110, as well as high-frequency noise and short spikes caused by the switching. These high-frequency components of the signal are filtered out by the first order low-pass filter that is provided by R0 and C. It is noted that the reference (+) terminal of the differential amplifier is at the output voltage Vout, meaning that the aforementioned voltage source is required to provide a supply voltage higher than Vout for the op-amp 131. As will be explained in the following, the PWM controller 120 is referenced to ground in this example, so that a DC-level shift of the signal VA is required in order to allow the voltage droop control circuit 130 and the PWM controller 120 to work together.
FIG. 3 shows details of the PWM controller 120, which may, as in the present example, comprise an output voltage regulator in the exemplary form of a PID regulator 121 optionally having a so-called “Lucent trim”. The PWM controller 120 also includes a switch drive circuit 122 that generates, based on the output of the PID regulator 121, drive signals for the switch(es) in the switch network 110 that is/are under its control. The signals Vout and VA input to the PWM controller 120 are referenced to a stable voltage reference, Vref, which is typically 1.25 V. The signal VA is fed to the inverting input of the op-amp 123 that forms part of the PID regulator 121. In FIG. 3, Vaux is an optional auxiliary “housekeeping” power supply for powering the internal electronics of the SMPS, and in this example acts to provide a bias current to the voltage reference diode during start-up when the Vout is not present. Vaux can also be smaller than the output voltage Vout.